Oxidative degradation of landfill leachate by catalysis of CeMnO_x/TiO₂ in supercritical water: Mechanism and kinetic study

Landfill leachate is a typical refractory wastewater that contains high concentration of organic pollutants. In this work, we investigated the treatment of landfill leachate by supercritical water oxidation (SCWO) in a batch reactor. The effects of temperature (T, 450–600 °C), oxidation coefficient (OC, 1.2–3.4), reaction time (t, 60–600 s) and pH (4.13–8.05) on total organic carbon (TOC) removal efficiency (TRE,%) and ammonia nitrogen (NH₃-N) removal efficiency (NRE,%) were analyzed. TRE and NRE increased remarkably with increasing temperature and OC. At 3.4 OC and 600 °C, 92.5% TRE and 50.9% NRE were achieved after 600 s. A modified kinetic model on TRE and NRE considering the induction time was developed and accurately correlated the experimental results. A significant co-oxidation effect of methanol and landfill leachate in SCWO was detected. Furthermore, heterogeneous catalysts were added to SCWO of landfill leachate to improve the removal rate of organic contaminants. A series of composite catalysts (CeMnO_x/TiO₂) which consisted of the cerium, manganese and titanium oxides were prepared by impregnation method with varying Ce/Mn ratio. Compared with the SCWO of leachate without catalyst, the CeMnO_x/TiO₂ catalyst with Ce/Mn ratio of 1:2 exhibited high catalytic activity and stability. Additionally, a catalytic reaction mechanism was proposed. It was assumed that MnO_x is the active component and CeO₂ is the additive and oxygen supplier. The results obtained from this work suggest the SCWO with high-efficiency and stable catalyst could be a promising technology for landfill leachate treatment.